1. Field of the Invention
The present invention generally relates to an optical reader for reading characters and/or graphic presentations which comprises an optical scanner having a position sensor, and more particularly, to a sheet-like pad for use with the optical reader. More specifically, the present invention relates to the sheet-like pad adapted to be laid below the optical reader for providing position information to the position sensor as the optical reader is moved above the sheet-like pad.
2. Description of the Prior Art
An optical reader of the type referred to above includes an optical scanner for reading, and providing an electric signal indicative of alphanumeric characters and/or graphic presentations depicted or reproduced on a sheet such as, for example, a sheet of paper. An example of this type of optical reader hitherto proposed is disclosed in, for example, Japanese Laid-open Patent Publication No. 1-500553 and U.S. Pat. No. 4,751,380.
The position sensor used in the prior art optical reader is an important tool to enable the optical reader to reproduce characters and/or graphic presentations accurately at required positions, by optically monitoring positional information imparted to the sheet or paper, when the characters and/or graphic presentations are printed out, displayed on a display device and/or stored in a data storage unit. In order for the positional information to be read by the optical scanner, a specially processed transparent film generally known as a sheet-like pad is utilized in association with the optical scanner.
The sheet-like pad referred to above has a grid pattern formed thereon or therein, having a plurality of equally spaced X-axis grid lines and a plurality of equally spaced Y-axis grid lines perpendicular to the X-axis grid lines. When in use, the sheet-like pad is laid beneath the optical scanner, particularly the position sensor, so that the latter can move above the sheet-like pad. As the position sensor is moved above the sheet-like pad traversing the grid lines, the position sensor counts the number of the grid lines which have been traversed thereby. The number of the grid lines so counted by the position sensor is converted into an electric signal representative of positional information which is subsequently issued by the optical scanner as a whole.
The sheet-like pad of the type used in the above described manner in association with the optical reader is required to satisfy the following requirements.
In the first place, the sheet-like pad must have a property that infrared rays of light emitted from the position sensor can be reflected by the sheet-like pad in a quantity greater than a predetermined value. Secondly, the grid pattern on the sheet-like pad should be such that a light responsive element sensitive to the infrared rays of light emitted from the position sensor can recognize the grid lines. In other words, the grid pattern on the sheet-like pad should include reflective and non-reflective areas alternating at predetermined intervals, said reflective areas being effective to reflect the infrared rays of light emitted from the position sensor. Finally, the sheet-like pad must have a transparency required for the optical scanner to receive rays of light, that is, must have a high light transmissivity with respect to visible rays of light.
An example of a prior art sheet-like pad hitherto designed to satisfy all of the foregoing requirements is schematically illustrated in FIGS. 12 to 14, reference to which will now be made for the purpose of discussion of the prior art believed to be pertinent to the present invention.
Referring first to FIG. 12 showing a perspective view of the prior art sheet-like pad 11, the sheet-like pad 11 is in the form of a film and has a periodic grid pattern of X-axis and Y-axis grid lines 12 forming square areas of uniform size thereon. FIG. 13 illustrates, on an enlarged scale, a portion of the sheet-like pad 11, and the grid lines generally identified by 12 are areas capable of absorbing infrared rays of light while the square areas of uniform size delimited by the grid lines 12 and generally identified by 13 are reflective area capable of reflecting the infrared rays of light.
As best shown in the sectional representation of FIG. 14, the sheet-like pad 11 is a multi-layered structure including a base layer 15 of polyester film, an infrared reflective layer 16, formed on one surface of the base layer 15 and capable of reflecting the infrared rays of light, and a protective layer 18. The infrared reflective layer 16 is of a multilayer structure including an Au-Ag alloy layer and an InO.sub.2 layer. The grid lines 12 forming the grid pattern are in the form of infrared absorbing layers 17 formed on one surface of the infrared reflective layer 16 opposite to the base layer 15 by the use of any known screen printing technique. The infrared absorbing layers 17 forming the grid lines 12 contain a dyestuff having an infrared absorbility extremely higher than its absorbability to the visible rays of light. The dyestuff contained in the infrared absorbing layer 17 may be a commercially available dyestuff such as, for example, IRA-870 manufactured and sold by EXCITON CHEMICAL of U.S.A.
The protective layer 18 is for the purpose of protecting square portions of the infrared reflective layer 16, which essentially form the square areas 13, and the infrared absorbing layers 17, which essentially form the grid lines 12, from damage which may be brought about by the contact of the optical reader with the sheet-like pad 11, and for this purpose, this protective layer 18 is formed on the infrared reflective layer 16 so as to overlay the infrared absorbing layers 17.
With the prior art sheet-like pad 11 so constructed as hereinabove described, as best shown in FIG. 14, infrared rays of light 20 emitted from the position sensor so as to be incident upon the sheet-like pad 11 at an angle of incidence of 30.degree. relative to the normal to the surface of the sheet-like pad are reflected in part by the non-reflective square areas 13, i.e., respective portions of the infrared reflective layer 16 which are not occupied by the infrared absorbing layers 17 forming the grid lines 12, and in part absorbed by the grid lines 12. On the other hand, visible rays of light 21 radiated by the optical scanner so as to be incident upon the sheet-like pad 11 at an angle of incidence of 45.degree. relative to the normal to the surface of the sheet-like pad 11 are allowed to pass through both of the infrared reflective layer 16 and the infrared absorbing layers 17. Accordingly, the visible rays of light 21 pass through the sheet-like pad 11 to reach a document 22 placed underneath the sheet-like pad 11 and are then reflected by the document 22. The visible rays of light 21 so reflected by the document 22 carry image information descriptive of alphanumeric characters and/or graphic presentations on one surface of the document 22 and, hence, form the imagewise rays of light. The imagewise rays of light reflected from the document 22 subsequently pass through the sheet-like pad 11, specifically through only the infrared reflective layer 16 or both of the infrared reflective layer 16 and the infrared absorbing layers 17, and are eventually detected by the optical scanner.
Thus, while the infrared rays of light radiated by a infrared light source built in the position sensor are repeatedly reflected and absorbed so that the infrared light receiver also built in the position sensor can provide an electric signal (binary signals) synchronized with a periodic structure of the grid lines 12, the counting of the binary signals generated from the infrared light receiver in the position sensor can result in a detection of the position of the optical reader on the document.
The prior art sheet-like pad 11 of the above described structure has been found to have problems. Specifically, since the visible rays of light 21 pass through the infrared reflective layer 16 at least twice with portion of the visible rays of light 21 passing additionally through the infrared reflective layers 17 as indicated by broken lines 21a in FIG. 14, attenuation of light takes place and, therefore, the efficiency of utilization of the rays of light tends to be lowered.
Also, in order for the infrared absorbing layers 17 forming the fine grid lines 12 to be formed on the infrared reflective layer 16 in the form of the periodic structure by the use of the screen printing technique, the screen printing is required to be repeated several times while work parameters such as, for example, the temperature of material for the infrared absorbing layers 17, the condition of the base layer 15 and the infrared reflective layer 16, the printing speed and the printing direction are strictly managed and controlled. Accordingly, it is not easy to manufacture sheet-like pads of uniform quality. It is eventually pointed out that the conventional screen printing is ineffective to provide the grid pattern of grid lines 12 which satisfies a required dimensional precision.
Furthermore, the formation of the infrared reflective layer 16 over the entire surface of the base layer 15 by the use of a coating technique requires an increased consumption of a coating material, resulting in an increased cost of manufacture of the sheet-like pad.